The present invention is concerned with the aminomethylation of the unsubstituted positions on the benzene ring of tocopherols, especially the complete aminomethylation of .delta.-tocopherol either alone or as a component of a mixture of several so-called "non-.alpha.-tocopherols" containing this tocopherol as well as, e.g., .beta.- and .gamma.-tocopherol.
It is known from the relevant literature that the conversion of .delta.-tocopherol, which differs from .alpha.-tocopherol by the presence of two unsubstituted positions (5- and 7-) on the benzene ring, into the corresponding 5,7-bis(aminomethyl) derivatives takes place only incompletely. An incomplete aminomethylation of .delta.-tocopherol leads, after the catalytic reduction performed subsequently in order to convert the aminomethylated product to .alpha.-tocopherol, to an .alpha./.beta.-tocopherol mixture which can only then be converted into .alpha.-tocopherol by an additional reaction cycle of aminomethylation+catalytic reduction. As an alternative to this additional cycle, the initially-produced .alpha.-tocopherol can be separated from the mixture of .alpha./.beta.-tocopherols, which of course likewise leads to an unsatisfactorily low yield of .alpha.-tocopherol, which tocopherol is preferred for known biological reasons over the non-.alpha.-tocopherols. For these reasons, the previously known processes for the aminomethylation of .delta.-tocopherol have been found to be expensive and accordingly uneconomical, which also applies to the aminomethylation of tocopherol mixtures containing .delta.-tocopherol.
Thus, Nakamura and Kijima [Chem. Pharm. Bull. 19(11), 2318-2324 (1971)] have reacted .delta.-tocopherol with in each case eight mol equivalents of aqueous dimethylamine solution and 37% formalin at reflux temperature for 4 hours and in this way obtained (see pages 2320 and 2322) a mixture of the 5-mono-substituted product, 5-dimethylaminomethyl-.delta.-tocopherol (57% yield), and the 5,7-disubstituted product, 5,7-bis(dimethylamino-methyl)-.delta.-tocopherol (only 31% yield). This result is obviously due to the fact that the 5-position of the .delta.-tocopherol molecule is substantially more reactive to aminomethylation than the 7-position and that the substitutability of the still free 7-position is further impeded by the introduction of the first amino substituent. The problem of incomplete aminomethylation is also evident in European Patent Publication (EP) 159 018 of the Henkel Corp., where the aminomethylation of a tocopherol mixture and the subsequent separation of the aminomethylated .beta.-, .delta.- and .gamma.-tocopherols from the unreacted .alpha.-tocopherol are disclosed. After catalytic hydrogenation of the aminomethylated tocopherols, the thus-obtained mixture of .alpha.-, .beta., .delta.- and .gamma.-tocopherols is again aminomethylated and hydrogenated in order to obtain (see pages 16-19) a product having a high as possible content of .alpha.-tocopherol. This second round of aminomethylation and hydrogenation comprises the aforementioned "additional reaction cycle" which the incomplete aminomethylation of the prior art required.